A common type of conventional mobile wireless communication system comprises a plurality of radio base stations, which are distributed over a certain geographical area. Communication with mobile end stations is transmitted through a local base station and on to a central network, which may be coupled to the public switched telephone network.
In such systems, a cell can be defined as the local geographical area, in which communication between a mobile end station and the base station in question can be carried out while meeting a set of predefined parameters. Such parameters could correspond to comparative measured signal strength values for signals transmitted between a mobile end station and a given base station and neighbouring base stations, respectively. The parameters may also relate to error rates. When so-called handover criteria are met, the mobile end station in question is handed over to a neighbouring base station.
Each base station is allocated a certain group of frequencies or communication channels, which are different from neighbouring cells. In this manner, it is accomplished that communication in a given cell is not disturbed by communication taking place in adjacent cells.
Many cellular systems have an inherent ability to direct traffic to more base stations for a given locality. This feature can be used to allocate the mobile end station in question to the base station which presently has free capacity or—in case of a base station failure—to direct a given mobile to a properly operating neighbouring base station. This redundancy enhances of course the reliability of the system.
However, radio spectrum is a sparse commodity and only a limited number of radio channels would normally be available to a given network provider. In order to utilise the allocated radio spectrum efficiently; frequencies or channels are typically re-used in a plurality of cells, which are situated at a certain minimum distance from one another.
Examples of such cell patterns wherein frequency re-use is implemented are for instance shown in prior art document WO98/35519.
For this type of cellular network, a certain frequency allocation plan for distributing allowed frequencies to the various cells are implemented at the planning stage of the network. Such frequency allocation plans are initially based on models, which may not correspond very well to reality. Therefore, they have to be revised once the network is put into operation. Several modifications to the frequency plan are typically necessary. The frequency plan may also be modified in order to take account of network expansion or changes in the physical conditions.
Many solutions exist for calculating appropriate cell plans. For instance prior art document EP-A-0 847 213 discloses a routine for assigning carrier frequencies to base stations in a non-interfering manner.
Methods for revising an existing cell plan are also known. Prior art document U.S. Pat. No. 5,603,085 discloses such a method.
The implementation of a new cell plan in a network is typically handled by the network operator by programming various operations in the network management system controlling the base stations in the network. During the implementation, the operations in question partly block large proportions of the network and the traffic related thereto. This leads to comparatively long outage times, decreased quality of service and lost revenues.